Method of making electric conductor

ABSTRACT

A method of making a conductive roving having a bundle of glass filaments and wherein the bundle has been flattened during curing into a ribbon, including drawing the flattened ribbon through a rotating orifice having a width less than the length of the ribbon, reforming the bundle into a cylinder and applying a false twist to the ribbon. The twisting of the bundle closely packs the filaments in a tight cylindrical bundle. The method further includes binding the cylindrical bundle by helically winding a non-conductive element under tension about the bundle, retaining the uniform cylindrical cross-section.

United States Patent Hurley Aug. 26, 1975 Inventor:

Assignce:

Filed:

Appl. No.:

METHOD OF MAKING ELECTRIC CONDUCTOR Raymond E. Hurley, Heath, OhioOwens-Corning Fiberglas Corporation, Toledo, Ohio June 17, 1974 US. Cl.57/162; 57/7; 57/140 G Int. Cl. D02g 3/02; D()2g 3/18 Field of Search57/3, 6, 7, 140 G, 156,

References Cited UNITED STATES PATENTS 6/1931 7/1940 ll/l956 12/1956Runquist ct al 57/160 X Shard ct al 57/140 G X Morrison et a1. 57/140 GX Whitchurst ct a1. 57/140 G X 3,644,866 2/1972 Dcardurff 57/140 G XPrimary Examiner-Donald E. Watkins Attorney, Agent, or FirmCarl G.Staelin; John W. Overman; Raymond E. Scott 5 7 ABSTRACT A method ofmaking a conductive roving having a bundle of glass filaments andwherein the bundle has been flattened during curing into a ribbon,including drawing the flattened ribbon through a rotating orifice havinga width less than the length of the ribbon, reforming the bundle into acylinder and applying a false twist to the ribbon. The twisting of thebundle closely packs the filaments in a tight cylindrical bundle. Themethod further includes binding the cylindrical bundle by helicallywinding a non-conductive element under tension about the bundle,retaining the uniform cylindrical cross-section.

5 Claims, 24 Drawing Figures PATENTEU M1925 v 380L016.

CREEL DRUM F163 2 a 3 FIGS. I8 T0 20 END COLLECTION.IMPREGNATION-OVERCOAT DIP.

FIGs. 2 a 3 FIGS. I8 T0 20 COATING. CURE.

FIGS. 2 a 3 FIGS. I8 TO 20 STRAND COLLECTION. OUTER OVERCOAT DIP.

FIGS. 2 a 3 I FIGS. I8 1020 cuRING. CURE.

FIGS. 4 a 5 FIGS. I8 T020 RIBBON BREAKING. RESISTANCE MONITORING.

FIGS. 7 T0 IO FIGS. I4 a I5 OVERWRAP. COLLECTION.

FIGS. I2 8 I3 FIGS. I6 8 I7 RESISTANCE MONITORING.

FIGS. l4 a I5 COLLECTION.

FIGS. I6 8 I? L FIG. I

PATENTED AUEZBISYS SHEET FIG. 3

PATENTEB AUBZBIQYB 3, 90 1,016

SHEET 3 10 PATENTED M325 1975 3. 9 01 ,O l 6 PATENTEDAUBZBISIS 3.901.016

SHEET 7 1O FIG. I? 228 PATENTEU M182 61975 SHEET FIG. 20

PATENTED 3,901,016

sum 10 1o FIG. 2 2

METHOD OF MAKING ELECTRIC CONDUCTOR FIELD OF THE INVENTION The conductormade by the method of this invention is particularly suitable for hightemperature and service applications which require uniform conductance,such as an automotive spark ignition cable. The method of making anelectric conductor of this invention permits the use of a continuoushigh speed line and results in an improved product which has uniformelectrical resistance. US. Pat. No. 3,247,020 of Shulver et al, assignedto the Assignee of the instant application, describes a method ofapplying an electrically-conductive coating to the outer surface ofglass fibers. In the method disclosed in the Shulver patent, the ends ofglass strands are collected from a creel, dipped in a thermally curableliquid dispersion of electricallyconductive particles and cured on aheated drum. The Shulver patent discloses various conductive coatingsand suitable compositions for the liquid dispersion and is incorporatedherein by reference. The conductive roving is then collected in theShulver patent on a takeup roll or reel. The method of this invention isparticularly adapted to improving the method disclosed in the Shulverpatent.

Further, as disclosed in a copending application for United Statespatent assigned to the Assignee of the instant application, Ser. No.322,311, filed Jan. 10, 1973 and incorporated herein by reference, theproblems of electrical interference with communications has resulted incertain government standards applicable'to automotive ignition cablesand the like. For example, the temperature within an automobile hood hasincreased steadily, due to larger horsepower engines and emissioncontrol devices, requiring greater temperature service capabilities forall engine components, including ignition cables. These requirementshave created an urgent need for ignition cables and the like having hightemperature service capabilities and a uniform conductance andresistance throughout its length, which are met by the electricalconductor made by the method of the present invention.

SUMMARY OF THE INVENTION The method of this invention is particularlyadapted to produce an electrical conductor having uniform resistancealong its length and uniform conductance between the core and asemi-conductive overcoat. The conductor includes an electricallyconductive core comprised of a bundle of electrically conductive glassfibers. The conductive core is retained in a cylindrical bundle by anoverwrap comprising one or more spirally wound elements, preferably of anonconductive material, such as glass. Finally, the conductive core isencased in an overcoat, preferably of a semi-conductive material such asa suspension of conductive particles in polytetrafluoroethylene. Theabove referenced copending application describes the advantages of thesemi-conductive overcoat, including the elimination of stray radiation,the ease of stripping, temperature ser vice capabilities and thepreferred low coefficient of friction.

The method disclosed herein optimizes the advantages f the conductordisclosed in the above referenced copending application. The dimensionsof the conductor are accurately controlled, including the diameter ofthe cylindrical core or roving and the semiconductive overcoat. Uniformresistance is assured by constant monitoring and control during theprocess. Uniform conductance between the core and the overcoat iscontrolled by first impregnating the conductive roving and then encasingthe roving and overwrap. For example, the method of this invention maybe used to produce an electrical conductor having a uniform resistanceof 4,000 i 500 ohms per foot. Further, the method of this invention maybe continuous, at relatively high speeds as compared with the prior art.

As described above, a primary object of the present invention is tomake'a conductive roving having uniform resistance along its length. Theresistance of the roving is however partially dependent upon having auniform circular cross-section.

In the overall method disclosed herein, the bundle of filaments areglass coated with a thermally curable liquid dispersion of conductiveparticles and the bundle is cured by wrapping around a heated drum ordrums under tension, flattening the bundle into a ribbon. Further, thecuring of the ribbon under tension may form a bond between thefilaments, making it diffucult to achieve uniform circularcross-section.

In the method of this invention, the flattened ribbon is drawn through arotating configured die orifice having a length greater than the widthof the ribbon and a width less than the ribbon width, breaking the bondbetween the individual conductive strands, reforming the strands into acylindrical bundle and applying a false twist to the roving. The falsetwist closely packs the filaments into a tight cylindrical bundle ofuniform crosssection. Finally, the cylindrical bundles of filaments isbound by helically winding an elongated element under tension about thebundle, forming an electrically conductive roving having a uniformcylindrical crosssection.

Other advantages and meritorious features of this invention will be morefully understood from the following description of the preferred methodsand the drawings, a description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart showing onemethod of this invention for vmaking an electrical conductor;

r FIG. 2 is a top elevation, partially schematic, showing a dippingstation for applying a thermally curable liquid dispersion of conductiveparticles to the roving.

FIG. 3 is a side view, partially cross-sectioned, of a dipping stationshown in FIG. 2;

FIG. 4 is a side elevation of a curing station;

FIG. 5 is a side elevation of FIG. 4, partially schematic;

FIG. 6 is a cross-sectional view of the conductive roving of FIG. 4 inthe direction of view arrows 6-6;

FIG. 7 is a partial side elevation of a breaking station which may beused in themethod of this invention;

FIG. 8 is a crosssectional view of the conductive roving shown in FIG. 7in the direction of view arrows 8-8;

FIG. 9-is a cross-sectional view of the conductive roving in thedirection of view arrows 9-9 of FIG. 7;

FIG. 10 is a cross-sectional view of the breaker apparatus shown in FIG.7, in the direction of view arrows FIG. 11 is a cross-sectional view ofFIG. 10, in the direction of view arrows 11-11;

FIG. 12 is a side elevation, partially cross-sectioned of an overwrapwinding station;

FIG. 13 is a cross-sectional view of FIG. 12 in the di rection of viewarrows 13-13;

FIG. 14 is a side elevation, partially schematic, of a resistancemonitoring station;

FIG. 15 is a partial top elevation of FIG. 14;

FIG. 16 is a side elevation of a winding station;

FIG. 17 is a top elevation of FIG. 16;

FIG. 18 is a side elevation, partially cross-sectioned of an overcoatdipping and curing station;

FIG. 19 is a perspective top elevation, partially schematic, of FIG. 18;

FIG. 20 is an enlarged view of a sizing and wiping die used in theapparatus of FIGS. 18 and 19;

FIG. 21 is a perspective end view of an electrical conductor formed bythe method of this invention;

FIG. 22 is a partially schematic side elevation of another embodiment ofthe method of this invention;

FIG. 23 is a side elevation, partially cross-sectioned, of anotherembodiment of an overwrap winding station; and

FIG. 24 is a top perspective view of another embodiment of an electricalconductor formed by the method of this invention.

DESCRIPTION OF THE PREFERRED METHODS OF THIS INVENTION FIG. 1illustrates one continuous method of this invention for making anelectric conductor, wherein a plurality of glass strands are collectedfrom a creel, coated with a thermally curable liquid dispersion ofelectrically conductive particles and cured, preferably on heatedrotating drums, forming the electrically conductive roving or group ofstrands into a flattened ribbon. The ribbon may then be pulled through abreaker, which breaks the bond between the strands, reforming the ribboninto a cylindrical roving and applying a false twist to the roving. Theroving is then bound in the cylindrical form with a spiral overwrap orthe overwrap may be applied directly to the flattened ribbon whichreforms the ribbon into the cylindrical roving. The electricalresistance of the overwrapped roving is preferably continuouslymonitored, which resistance is a function of the curing of the strandcoating and therefore the temperature of the heated drums. An errorsignal may thus be produced at the monitoring station, which is used tocontrol the temperature of the drums, thereby controlling the criticalresistance of the electrical conductor formed by the method of thisinvention. By this method, the electrical resistance of the conductormay be accurately controlled within relatively narrow limits. Theconductor may then be collected on a suitable drum in a collectionstation for later use.

The electrical conductor is then impregnated with an overcoat material,preferably a semi-conductive material such as a liquid dispersion ofcarbon particles in tetrafluoroethylene. The impregnated roving is thenpartially cured, dipped again in the overcoat material and finallycured. In the preferred method, the resistance of the semi-conductiveovercoat material is monitored in a second resistance monitoring stationand finally the electric conductor is collected on a drum or the like.The electrical resistance of the semi-conductive overcoat is a functionof the percent of conductive particles, such as carbon, in the overcoatmaterial and therefore the resistance may be controlled during thecontinuous operation by increasing or decreasing the percentage ofconductive particles. The method steps shown in the flow chart of FIG. 1refer to the Figures of this application wherein the method steps aredisclosed in detail. As will be described herein, the method steps ofthis invention may be combined and modified in various ways to make anelectrical conductor or cable and FIG. 1 illustrates only one suchcombination.

FIGS. 2 and 3 illustrate one method of collecting and coating the glassstrands utilized in the method of this invention. The method of thisinvention is particularly concerned with the manufacture of anelectrical conductor having a cylindrical core or roving comprised of aplurality of glass strands which are initially nonconductive, such asdescribed in the above referenced United States patent of Shulver et al.The strands may be formed of any suitable material, such as E Glassdisclosed in US. Pat. No. 2,333,961, assigned to the Assignee of theinstant application. A strand is defined for the purpose of thisapplication as a bundle or plurality of glass filaments.

In FIG. 2, the strands 42 are collected from a creel 40 having aplurality of rolls 44. A suitable creel is disclosed in the abovereferenced United States patent of Shulver et al. A plurality of strands42 are then collected in an eye 48. In the method of FIGS. 2 and 3, sixsets of ten strands are collected in eyes 48 and dipped in a thermallycurable liquid dispersion of conductive particles, as described. Thenon-conductive groups of strands 46 are then passed beneath dippingroller 52 in tank 50. The wetted strands are then passed between thepressure or squeeze rollers 54 and are finally collected in the form ofa roving 76 in wiping die 56. In the disclosed method, a roving is agroup of strands which forms the core of the electrical conductor. Aroving may also be formed of a grouping or bundle of individual glassfilaments.

The thermally curable liquid dispersion of conductive particles entersthe tank 50 in the disclosed apparatus through inlet pipe 58 fromreservoir 60. The tank includes a bottom wall 62, side walls 64 and oneend wall 66. The tank end 68 is open to permit continuous flow of theliquid dispersion and prevent settling of the conductive particles. Theliquid dispersion used in the dip tank 50 of FIGS. 2 and 3 is preferablyrelatively viscous, such that the liquid collects on the bottom of thetank and flows out of the open end 68 into a funnelshaped trough orcollector 70. The collector includes side walls 72 and a return line 74to the reservoir 60. The flow of liquid is controlled such that thelevel is above the bottom of the dip roller 52 and below the level ofthe wiping die 56 and the level of the strands 46, between the pressurerollers 54. The wiping die has a predetermined size opening, wiping theexcess of fluid from the roving 76, referred to herein as a roving andlater as the core of the electrical conductor made by the method of thisinvention. A suitable size for the opening of the wiping die 56, whereinsixty strands are collected in the die, is 0.065 inches. The number ofstrands will of course depend upon the particular application for theconductor, however a suitable roving 76 has sixty glass strands, eachstrand having a diameter of about 0.005 inches. Various thermallycurable liquid dispersions of conductive particles may also be utilizedin the method of this invention, such as disclosed in the abovereferenced United States patent of Shulver et al.

For example, a dispersion of particles of graphite in a water solublethermosetting resin may be used having 7 to 12% by weight solids,preferably graphite. The resin serves as a binder for the graphiteparticles and adheres to the strands. As described in the Shulver et alpatent, the coating may be carbonizable, pyrolizable or caramelizable.As used herein, a thermally curable liquid dispersion of conductiveparticles refers to a liquid which may be cured to provide anelectrically conductive roving as described in the Shulver patent. Onepreferred method of drying the roving 76 and forming the electricallyconductive core is disclosed in FIGS. 4 and 5.

As shown in FIGS. 4 and 5, the roving 76 is initially generallycylindrical because the aperture in the wiping die 56 is cylindrical.The wetted roving is then tensioned about a pair of heated rotatingdrums 80. In FIG. 5, the roving takes ten passes over the drums. Thedrums 80 are supported for rotation on shafts 82 which are connected tomotor 86 by a suitable drive means 84. The drive shafts 82 are supportedin suitable support blocks 90.

The drums 80in FIGS. 4 and 5 are electrically heated by Chromalonelectric heater elements 92 which are enclosed heating elements, such assold by E. L. Wiegand Division of Emerson Electric Company. Each of thedrums in the disclosed embodiment includes seven heater elements whichare individually controlled to heat the drums as described below. Theroving is preferably tensioned around the drums, as shown in FIGS. 4 and5 and the drums pull the roving through the dip tank 50 in FIGS. 2 and3. The heated drums dry the roving and bake the electrically conductivecoating. As described in the above referenced United States patent ofShulver et al, the coating applied in the dip tank 50 may becarbonizable, pyrolizable or caramelizable, wherein an increase in heatlowers the electrical resistance of the conductive roving. Theelectrical resistance of the roving may thereby be controlled bycontrolling the temperature of the heated drums as will be describedhereinbelow. The temperature of the drums is normally maintained betweenabout 650 and l 100F.

As described above, the conductive roving is preferably tensioned aboutthe drums 80 and the rotating drums pull the roving through the diptank. The tension in the roving thereby flattens the roving into aribbon 768 as shown in FIG. 6. The flattening of the ribbon improves thethermal contact between the roving and the heated drums, reducing therequired number of passes of the ribbon around the drums and thereforethe time of curing. The roving is thereby worked into the flattenedribbon 76B shown in FIG. 6, however in the preferred embodiment of theelectrical conductor made by the method of this invention, theconductive roving or core of the electrical conductor is cylindrical asdescribed hereinbelow. As shown in FIG. 5, the rotational axis of thedrums 80, as defined by the drive shafts 82, have converging axes, suchthat the tensioned roving normally slides sideways toward the end of thedrums as the roving is pulled into the next station. By this method, theroving is received around the drums at one end and is removed afterseveral passes from the opposed ends without requiring a guide means orthe like. The drums may therefore have a smooth surface providingexcellent thermal conductivity. One method of reforming the flattenedribbon of conductive fibers into a cylindrical roving is shown in FIGS.7 to 11, which is referred to as the breaker stations.

The breaker assembly includes a drive 102 having a driven shaft orspindle 104 interconnected to a motor 106 through belt 108 and pulleys110 and 112. The spindle 104 includes a breaker insert 114 which rotateswith the spindle and is secured thereto by a suitable set screw 116 orthe like.

The breaker insert 114 includes a configured orifice or opening 1 18which receives the flattened ribbon 76B and reforms the strands into acylindrical roving as described below. The insert opening 118 includes aconical inlet 120 and a smaller diameter conical outlet 122, as shown inFIG. 1 1. The configured opening 1 18 of the insert 114 is preferablyelongated, as shown in FIG. 10, having a width x less than the width aof the flattened ribbon, as shown in FIG. 8. The length y of the openingis greater than the width a of the ribbon. As shown, the width x of theopening is greater than the thickness b of the ribbon. In the examplethus far described, the ribbon may have a width a of 0.065 inches and athickness of 0.041 inches. A suitable insert opening would then have alength y of 0.125 inches and a width x of 0.055 inches. The cornerradiuses may be 0.025 inches for example.

As described above, the insert 114 rotates with the spindle 104 and theflattened ribbon is drawn through the opening 118 of the insert. Therotating orifice will thus break the bond between the individual strandsand reform the strands into a cylindrical roving as shown at 76C in FIG.9. Further, the elongated opening will apply a false twist to theroving, downstream of the breaker insert, closely packing the strands inthe roving. The necessity of a breaker will then depend upon thestrength of the bond between the individual strands, which is formedduring the curing of the conductive coating upon the heated drums shownin FIGS. 4 and 5. In the preferred embodiment of the method of thisinvention, the strands of the conductive roving are bound by a spirallywound overwrap, which is sufficient in many applications to reform theflattened ribbon into a cylindrical bundle. The breaker however hasimportant advantages in the method of this invention, including theapplication of a false twist which closely packs the strands in thecylindrical bundle, as described. The breaker should however beconsidered optional in the overall method, although important to themethod of this invention.

FIGS. 12 and 13 illustrate one method of applying the overwrap whichbinds the conductive strands into a cylindrical roving, which forms theconductive core of the electrical conductor of this invention.Altemative methods will be disclosed in the description of FIGS. 21 to23. In the overwrap winding station 130, shown in FIGS. 12 and 13, theconductive roving bundle 76C is received in a stationary hood or shroud132. The hood shown in FIG. 12 is cup-shaped and prevents transient aircurrents from disturbing the winding operation described below. Incertain applications therefore the preferred hood may be eliminated.Inside the hood is a rotating bobbin or spool 134 having yarn or astrand of a non-conductive fiber 136 wound thereon. A suitable yarnwould be a continuous twisted strand of fiber glass, such as E Glassdescribed above. The bobbin includes opposed rims 138 which retains theyarn on the bobbin and the bobbin is secured to a hollow drive shaft orspindle 140 for rotation therewith. The

drive shaft is connected through a suitable bearing 139 in support 141to motor 142 by belt 144 and pulleys 146.

In the disclosed embodiment, the bobbin 134 is hollow as shown in FIG.13, wherein the rims 138 are connected to the drive shaft 140 by aplurality of fins 150. The fins 150 may generate further air currents inthe hood 132 and aid in the control of the ballooning yarn 162 as willnow be described. The rotating bobbin generates air currents indicatedby the arrow 160 which enters the hood, circulates over the bobbin andlifts the ballooning yarn as shown. The conductive roving 76C is pulledthrough the axial bore 140 of the bobbin and the yarn is wound undertension on the roving, as shown at 164 in FIG. 12. As described, thebobbin is rotating at a sufficient speed to throw the yarncentrifugally, preferably in an unconfined balloon 162 having its freeend adhered to the roving. The centrifugal force of the balloon is thusused to wind the yarn on the roving under sufficient tension to bind theroving into a cylindrical form. In the disclosed embodiment, the fins150 may further generate air currents which pass through the bobbin anddirect the currents within the hood 132 as shown. The yarn in thisembodiment of the overwrap station may be further given a twist as itunwinds from the bobbin and is spirally wound under tension on theconductive roving. The twist will depend upon the rotational speed ofthe bobbin and the speed of the line. In another application of themethod of this invention, the yarn is wound on the roving prior toreceipt in the bobbin, as will be described in regard to FIG. 23. In themethod disclosed in FIG. 1, the electrical resistance of the overwrappedconductive roving 76D is then continuously monitored to control thecuring of the electrically conductive surface, such as by the methoddisclosed in FIGS. 14 and 15.

A suitable resistance monitoring station 170 is shown in FIG. 14,wherein the conductive roving 76D is directly in line with the overwrapwinding station or set up as a separate station including feed drum 172.It will be understood that the preferred method of this invention iscontinuous.

The resistance monitoring station includes a pair of spring biasedelectrically conductive rollers 174 and 176 which tension the conductiveroving 76D between the rollers for more accurate measurement of theresistance. The rollers are each mounted on a shaft 178, as shown inFIG. 15, which are rotatably received in opposed spring biased mountingblocks 180. The mounting blocks are each slidably supported on shafts182 having a helical spring 184 biased between the mounting blocks 182and a collar 186. A top collar 188 retains the blocks on the shafts 178.The pulleys 174 and 176 are thus continuously biased against theconductive roving 76D, tensioning the roving between the pulleys andaccurately controlling the distance between the points of contactbetween the pulleys and the roving, as will be described more fullybelow. In the disclosed embodiment, the support shafts 178 are connectedto and supported by a suitable support 190 by connectors 192, such asnuts, collars, etc.

The pulleys are formed of a conductive material and have in thedisclosed embodiment an integral metal hub 194. The hubs thereby providea direct electrical connection to the conductive roving 76D throughpulleys 174 and 176. Electrical connectors 196 are secured to eachmounting block 180, to move with the pulleys and include an electricalcontact brush 127 which electricallycontacts the integral hub 194 ofeach pulley. The connectors are connected by wires or cables 198 and 200to a monitor 202 shown schematically in FIG. 14. The monitor includes astandard recording ohm meter, such as sold by EsterlineAngus Company,Model A601C. As will be understood by one skilled in the art, therecording ohm meter is then connected to limit control switches, whichin turn are connected through relays to the heater elements 92 shown inFIG. 4. A voltage is then impressed by electrical connector 196, throughwire 198, to pulley 174 and the resistance drop of the length ofconductive roving 76D across the pulleys is measured and recorded by theohm meter of monitor 202. This is compared with a standard. As describedabove, if the resistance is below a predetermined minimum, thetemperature of the drums in FIGS. 4 and 5 is automatically increased,lowering the resistance of the conductive roving. If the measuredresistance is greater than a predetermined maximum, the temperature isdecreased, which will increase the resis tance of the conductive roving.The temperature of the drums is controlled by the heating elements 92which are electrical heating elements located within the drums asdescribed above. When the temperature of the drums is to be increased,for example, a limit switch is closed, closing appropriate relays whichactuates additional heating elements within the drums. The electricalcontrol is available commercially.

The method of controlling the electrical resistance of the roving thenincludes (1) thermally curing the conductive coating, as by winding theroving on the heated drums 80 in FIGS. 4 and 5, (2) continuouslymonitoring the resistance of the roving, as shown in FIG. 14 and (3)controlling the curing, as by continuously controlling the temperatureof the drums. In the method disclosed above, the strands 46 are coatedwith a thermally curable dispersion of conductive particles, however asdescribed above, the coating may be carbonizable, pyrolizable orcaramelizable. In the method of monitoring the resistance of the rovingshown in FIG. 14, the spacing between the rollers 174 is accuratelycontrolled, such that the resistance of a predetermined length of rovingis continuously monitored. For example, the distance between thecontacting points of the rollers may be accurately controlled to onefoot, such that the resistance of a one foot length of conductive rovingis continuously measured and compared to a predetermined resistance,which is the desired resistance of the electrical conductor. Thedisclosed method of monitoring the resistance of the roving thenincludes (1) continuously conducting a current along a predeterminedlength of the thermally cured strands, (2) continuously measuring thevoltage across the predetermined length and (3) continuously comparingthe voltage drop with the desired voltage drop. Finally, the extent ofcuring of the strands is varied when the voltage drop varies from thedesired resistance by controlling the temperature of the drums. Asdescribed above, this is accomplished continuously and automatically byactivating the heating elements 92 within the heat drums.

The electrically conductive roving may then be wound on a drum 204 andstored for later use in the method of making an electrical conductor ofthis invention, or used in other applications. A suitable windingstation 210 is shown in FIGS. 16 and 17. In the disclosed embodiment ofthe winding station, the conductive roving 76D is received around drivenpulleys 212 and 214, providing a capstand type pulling action,tensioning the conductive roving. The pulleys are drivably mounted ondrive shafts 216 and 218, respectively. The drive means includingmotors, pulleys, etc. are located within housing 219.

The conductive roving is then fed onto the take-up drum 226 through aconventional traverse 220 mounted on shaft 222 and guide key 224. Asuitable winding station is available from Davis Electric Company.

In the continuous method of this invention shown in FIG. 1, the windingstation pulls the conductive roving against the tension of the drums 80in FIGS. 4 and 5, providing a relatively high speed continuous processof making a conductive roving. Further, in the method disclosed in FIG.1, a semi-conductive overcoat may then be applied by the method shown inFIGS. 18 to 20, as described below.

In the overcoat station 234 shown in FIGS. 18 and 19, the conductiveroving 76D is received from a feed drum 238, over pulley 240 and intodip tank 236. In the preferred method of the invention, the tank 236contains a liquid polymeric dispersion of conductive particles, such asmore fully disclosed in the copending application for United Statespatent assigned to the Assignee herein, Ser. No. 322,311, filed Jan. 10,1973, now U.S. Pat. No. 3,818,412. The semi-conductive materialdisclosed in such application, which is incorporated herein byreference, comprises a suspension of fine conductive powders, preferablygraphite or carbon, in polytetrafluoroethylene. Tetrafluoroethylene isavailable commercially from E. I. duPont de Nemours Company under thetradename Teflon. The graphite carbon particles are added to the Teflonas a dispersion, which may also include a filler, such as a silicate,wetting agents, etc. A suitable graphite dispersion, for example,includes 100 parts by weight tetrafluoroethylene as received, 27.3 partsby weight graphite and 62.7 parts by weight water.

The conductive roving 76D is, first impregnated with the semiconductivefluid as shown in FIGS. 18 and 19 by dip pulley 242. The impregnatedconductive roving 76E is then partially cured in furnace 246. The rovingenters the furnace through opening 244 and leaves the furnace throughexit 248. The partially cured roving is cooled in this embodiment of themethod outside the furnace, being received around pulleys 250, 252 and254. The impregnated roving is then dipped in the tank 236 around dippulley 256 and cured in furnace 246. In the disclosed embodiment, theovercoated roving 76F is disposed around pulley 258 located within thefurnace and out of the furnace, around pulley 260. The semi-conductiveovercoat is thus fully cured within the furnace.

In the disclosed embodiment of the dip tank 236, wiping and sizing dies262 and 264 receive the conductive roving as it leaves the tank. Detailsof the sizing die 262 are shown in FIG. 20, which includes an annulardie member 266 and a wire frame 268. The die member 266 is looselyreceived on the conductive roving and normally rests upon the spacedparallel lower frame fingers or arms 272. When an agglomeration of fluidcollects on the roving, the die member will lift from the lower arms toengage the resilient upper arms 272, removing the accumulation of fluidfrom the roving and simultaneously sizing the coating. The diameter ofthe die opening 274 will depend upon the particular application. Forexample, in the embodiment of the electrical conductor disclosed above,a suitable die opening for the first die is 0.060 inches, wherein theroving has been impregnated with the semi-conductive overcoat material.The second die would then be 0.065 inches, which sizes the overcoat.

The electrical conductor 280 in FIG. 21, formed by the method of thisinvention as described above, then includes an electrically conductiveroving 282 comprising a plurality of conductive strands, a spiraloverwrap 284 of a non-conductive yarn or strand and preferably asemi-conductive overcoat 286. As described in the above referencedcopending application, the overwrap 284 is preferably a distinct windingor windings, rather than a braid, which provides uniform spacing betweenthe windings. The spacing is accurately controlled and the overwrap iswound under tension, such as a tension of 50 grams, to secure theconductive roving in a uniform cylindrical bundle. The electricalconductor formed by the method of this invention has a uniformcross-section and the overwrap provides a uniform conductance betweenthe roving 282 and the semiconductive overcoat 286. In the preferredmethod of forming the overcoat 286, the roving is first impregnated withthe semiconductive material, filling the nonconductive spaces betweenthe non-conductive yarn and the individual strands of the roving 282.Finally, a uniform layer or overcoat 286 is formed over the impregnatedroving providing the preferred uniform conductance between thesemi-conductive overcoat and the conductive roving. This is particularlyimportant where a uniform resistance is to be obtained in the electricalconductor, which is a primary object of the method of this invention.

In the method disclosed in FIG. 1, the resistance of the semiconductiveovercoat 286 is monitored continuously in a monitoring station, such asshown at in FIG. 14. In the second monitoring station however themonitor 202 is a recording ohm meter and the resistance is compared withan acceptable standard. Where the resistance is too high, the weightpercent of conductive particles is increased in the dip tank 236,reducing the overcoat resistance to the desired value. Where theresistance is too low, the weight percent of the binder is increased.Finally, the electrical conductor formed by the method of this inventionmay be wound on a drum at a second winding station, which may be similarto the winding station shown at 210 in FIGS. 16 and 17.

FIG. 22 illustrates an alternative method of forming a conductiveroving. Some of the apparatus may be identical to the apparatuspreviously described, as set forth below. In the method shown in FIG.22, the strands 302 are collected in an eye 304 to form a plurality ofstrands 306, as described in regard to FIGS. 2 and 3. The strands arethen dipped in a thermally curable liquid dispersion of conductiveparticles at dipping station 308 and collected in a single roving orcore as described above. The roving is then cured at curing station 312on rotating drums 310 as described above in regard to FIGS. 4 and 5. Theroving 309 now has been flattened into a ribbon as shown at 763 in FIG.6. In this embodiment of the method, the breaking station has beeneliminated and the conductive roving is reformed into a cylindricalroving or core in the overwrap winding Stations 314 and 316.

As described above, the overwrap is wound on the roving under tensionand may be utilized to reform the roving into a cylindrical core. Inthis embodiment of the method, two non-interwoven windings are used tosecurely retain the conductive roving in a cylindrical bundle of uniformcross-section. 318 designates the roving with a single winding and 320designates the roving with a double winding.

The second winding station 316 differs from the winding stationdisclosed in FIGS. 12 and 1 3 in that the yarn or strand is wound on theroving prior to receipt in the spindle, as more clearly shown in FIG.23. The overwrap winder includes a hood or shroud 330 and a rotatinghollow bobbin 332 located within the hood. The bobbin includes theoverwrap yarn 334 and rims 336. The bobbin is preferably hollow asdescribed above and the rims 336 may include fins as shown at 150 inFIG. 13. The bobbin is supported on a shaft or hub 338 which isconnected to a motor by belt 342. The details of the winding station maytherefore be the same as shown in FIGS. 12 and 13 except as described.

The direction of travel of the conductive roving 309 has been reversedin FIG. 23. The roving enters the open end of the shroud 330 and theyarn is thrown in a balloon 334 to spirally wind on the conductiveroving prior to entering the bobbin 332. By this method, the overwrapwinding and tension may be more accurately controlled, making a tighterbundle and a rounder product. Also, the bobbin is self-cleaning. Thespeed of the line and rpm of the bobbin may also be increased, forexample to 8100 to 12,000 rpm, providing 4 to 6 turns per inch ofconductive roving. Further, the tension is more uniform, which isparticularly important in providing a roving having uniform resistance,as described above.

The remainder of the method may be the same as described above. Theresistance of the conductive roving may be continuously monitored on aresistance monitoring station, such as shown at 170 in FIG. 14,permitting the continuous adjustment of the resistance of the roving bycontrolling the temperature of the drums during the forming operation.Finally, the conductive roving may be coated with a semiconductiveovercoat in the coating and curing station shown at 234 in FIGS. 18 and19. The conductive roving formed by this method is shown in FIG. 24.

The conductor shown in FIG. 24 includes a conductive roving or core 352comprising a bundle of conductive strands of glass fibers. The strandsare uniformly and tightly secured by two non-interlacing spiral windings354 and the overwrap is covered with a semiconductive overcoat 356.

As described in the above referenced copending application for UnitedStates patent assigned to the Assignee herein, the overwrap includes twodistinct winding layers, rather than a braid, which is easier to stripand provides more uniform spacing between the windings. The spacing ispreferably controlled to between 1/ 16 to 3/ 16 inches, measured betweencross-over points, which, in combination with the semi-conductiveovercoat, provides uniform conductance between the overcoat and thecore.

The steps of the method of this invention may therefore be modified andcombined to provide a continuous process for making an electricalconductor. Further, the .method of this invention produces an electricalconductor having uniform resistance within relatively narrow limits. Forexample, an electrical conductor having a resistance of 4,000 i 500 ohmsper foot may be formed by the method of this invention, which isparticularly important in many applications of the electrical conductor.

I claim:

1. In a method of making a conductive roving having a plurality ofconductive glass filaments, including curin g a liquid dispersion ofconductive particles on a bundle of glass filaments, under tension andflattening the bundle into a ribbon, the improvement comprising drawingthe flattened ribbon through a rotating noncircular orifice, saidorifice having at least one dimension less than the width of the ribbon,applying a false twist to the ribbon and making the ribbon generallycylindrical.

2. The method defined in claim 1, including binding the bundle offilaments in its cylindrical shape by helically winding an elongatedelement about the bundle under tension.

3. In a method of making a conductor, wherein a plurality of glassfilaments are coated with a thermally curable liquid dispersion ofconductive particles and then drawn into a single roving, the steps ofwinding said roving under tension about a heated drum, flattening saidroving into thermal contact with the drum and curing the liquiddispersion, said roving now having a width greater than its thicknessand finally drawing said flattened roving through a rotating die openinghaving 4. The method defined in claim 3, including the additional stepof helically winding a strand under tension about the cylindricalroving, securely retaining the conductive strands and forming a uniformcylindrical cross-section.

5. In a method of making a conductive roving, wherein a bundle ofconductive filaments have been formed into a relatively fiat ribbonhaving a width greater than its thickness, comprising the steps ofpulling the ribbon through a rotating die orifice having a lengthgreater than the width of the ribbon and a width less than the ribbonwidth, reforming the filaments into a cylindrical bundle and applying afalse twist and binding the cylindrical bundle of filaments by helicallywinding a non-conductive elongated fiber under tension about the bundle,forming a uniform cylindrical cross-section.

1. IN A METHOD OF MAKING A CONDUCTIVE ROVING HAVING A PLURALITY OFCONDUCTIVE GLASS FILAMENTS, INCLUDING CURING A LIQUID DISPERSION OFCONDUCTIVE PARTICLES ON A BUNDLE OF GLASS FILAMENTS, UNDER TENSION ANDFLATTENING THE BUNDLE INTO A RIBBON, THE IMPROVEMENT COMPRISING DRAWINGTHE FLATTENED RIBBON THROUGH A ROTATING NON-CIRCULAR ORIFICE, SAIDORIFICE HAVINGG AT LEAST ONE DEMESION LESS THAN THE WIDTH OF THE RIBON,APPLYING A FALSE TWIST TO THE RIBBON AND MAKING THE RIBBON GENERALLYCYLINDRICAL.
 2. The method defined in claim 1, including binding thebundle of filaments in its cylindrical shape by helically winding anelongated element about the bundle under tension.
 3. In a method ofmaking a conductor, wherein a plurality of glass filaments are coatedwith a thermally curable liquid dispersion of conductive particles andthen drawn into a single roving, the steps of winding said roving undertension about a heated drum, flattening said roving into thermal contactwith the drum and curing the liquid dispersion, said roving now having awidth greater than its thickness and finally drawing said flattenedroving through a rotating die opening having a width less than the widthof said roving, said die opening imparting a false twist to said roving,breaking the bond between the filaments and making the roving generallycylindrical.
 4. The method defined in claim 3, including the additionalstep of helically winding a strand under tension about the cylindricalroving, securely retaining the conductive strands and forming a uniformcylindrical cross-section.
 5. In a method of making a conductive roving,wherein a bundle of conductive filaments have been formed into arelatively flat ribbon having a width greater than its thickness,comprising the steps of pulling the ribbon through a rotating dieorifice having a length greater than the width of the ribbon and a widthless than the ribbon width, reforming the filaments into a cylindricalbundle and applying a false twist and binding the cylindrical bundle offilaments by helically winding a non-conductive elongated fiber undertension about the bundle, forming a uniform cylindrical cross-section.